Method of phase advancing compensation control for an AC synchronous motor

ABSTRACT

An AC synchronous motor control method in which the optimum phase advancing control is executed not only in acceleration but also in deceleration of a motor. When a value P, which is obtained by multiplying a motor&#39;s revolutionary speed by a torque command, is positive or equal to &#34;0&#34;, a proportional constant of a linear equation for calculating a phase advancing compensation amount δ is set to k, whereas, when the above value P is negative, the above proportional constant is set to k&#39; (k&#39;&lt;k). Then, the phase advancing compensation amount δ is calculated by using the proportional constant k or k&#39;, and the calculated compensation amount δ is added to a rotor phase θ, thus advancing the phase of current commands. The motor is controlled by executing current loop processing on the basis of a current command T thus obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control method of an AC synchronousmotor, in particular, to a phase lead control method.

2. Description of the Related Art

In controlling an AC synchronous motor, as the revolving speed of themotor increases to enter high speed range, the power factor of the motortends to fall due to the phase delay of the supplied current. In orderto prevent the fall of the power factor, a phase advancing compensationcontrol, which controls the phase of a current command by advancing thephase, is generally employed.

FIG. 1 is a block diagram showing a current control system for executingthe phase advancing compensation control. In FIG. 1, a phase of acurrent command (torque command) Tcmd is advanced by δ using a phaseadvancing module 1, and the current command T, whose phase is advanced,is treated as a current command for a current loop. According to currentloop processing, the difference between the current command T, whosephase is advanced, and an actual current I of a motor is calculated, andthe calculated difference is integrated by an integrating module 2, andis further multiplied by an integrating constant k1. A proportionalmodule 3 multiplies the actual current I by a proportional constant k2.An output value of the proportional module 3 is subtracted from anoutput value of the integrating module 2, and a counter electromotiveforce correction value kEo is added to the difference. The value thus.obtained is treated as a command voltage (terminal voltage) Vc suppliedto the motor.

A module 4 is a term representing a motor coil. A voltage actuallyapplied to the motor coil (R and L represent resistance and inductance,respectively) is a voltage obtained by subtracting a counterelectromotive force Eo from the aforesaid command voltage (terminalvoltage) Vc, and a current I flows.

According to the aforesaid phase advancing module 1, the phase advancingcompensation amount δ is calculated by a linear function proportional toan absolute value of the motor's revolutionary speed v, and isordinarily obtained by a function expressed by the following equation(1):

    δ=K·|v|                   (1)

Where, K is a proportional constant.

Using the phase advancing compensation amount δ and based on theposition of the rotor, i.e., based on the counter electromotive force Eoand the current command Tcmd, the phase-advanced current command T (avector) can be expressed as follows:

    T=[To·cos δTo·sin δ].sup.T   ( 2)

Where Tcmd=[To 0]^(T)

The actual current I (vector) is given as follows:

    I=[x1·To y1·To].sup.T                    ( 3)

In addition, the following equation (4) is obtained on the basis of therelationship between the command voltage applied to the motor coil andthe voltage applied to the coil side as is shown in FIG. 1.

    (k1/jω)(T-I)-k2·I+k·Eo=Eo+(R+jωL)I(4)

If a current vector [x1 y1]^(T) is calculated by the above equation (4),the following equation (5) is obtained. ##EQU1##

FIGS. 3 through 6 are vector diagrams drawn on the basis of the aboveequations (4) and (5). FIGS. 3 and 4 show vector diagrams in the casewhere the motor is accelerated at a velocity of 2000 rpm. FIG. 3 shows avector diagram in the case where the phase advancing compensation amountδ is set to "0" (zero). FIG. 4 is a vector diagram in the case where thephase advancing compensation δ is set to 99.7 degrees. In FIG. 3, theactual current I is delayed by θ1 with respect to the current commandTcmd (counter electromotive force Eo); on the other hand, the commandvoltage Vc becomes more than a DC link voltage which is a power-supplyvoltage of an inverter. Therefore, the power factor becomes poor. InFIG. 4, the command voltage Vc is within the DC link voltage, and powerfactor becomes better. In other words, the power factor is improved bythe phase advancing control.

FIGS. 5 and 6 are vector diagrams in the case where the motor isdecelerated at a velocity of 2000 rpm. FIG. 5 shows a vector diagram inthe case where the phase advancing compensation amount δ is set to "0"(zero). FIG. 6 shows a vector diagram where the phase advancingcompensation amount δ is set to 99.7 degrees. As is obvious from thecomparison between FIGS. 5 and 6, when the phase advancing control isexecuted, the command voltage Vc becomes higher than the DC link voltagewhich is a power-source voltage of the inverter for controlling themotor. This means that a voltage cannot be applied up to the commandvoltage Vc. In other words, if the phase advancing correction is equalto that in the case of the acceleration, the phase correction becomesexcessive.

SUMMARY OF THE INVENTION

The present invention provides a motor control method by the optimumphase advancing control both in accelerating and in decelerating.

The method according to the present invention comprises a step ofdetermining whether a motor is to be accelerated or decelerated on thebasis of the relationship between the motor's actual revolutionary speedand a current command, and a step of advancing a phase of the aforesaidcurrent command only by the amount of phase of advancing correction. Ifthe motor is determined to be decelerated, the aforesaid amount ofcompensation is set to a value smaller than the amount of compensationin accelerating. The aforesaid amount of phase advancing compensation iscalculated by a linear, equation proportional to the motor'srevolutionary speed. When the motor is being decelerated, a proportionalconstant of the above linear equation is set to a constant smaller thanthat in the case of accelerating.

By setting the amount of phase advancing compensation during thedeceleration of the motor to an amount smaller than that during theacceleration, the command voltage Vc applied to the motor is within theaforesaid DC link voltage even during deceleration, so that not only thepower factor is improved but also an adequate output torque is obtainedin the motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional current control forexecuting a phase advancing correction control of a current command;

FIG. 2 is a flowchart of current command computing processing carriedout by a processor of a motor controller in one embodiment according tothe present invention;

FIG. 3 is a vector diagram in the case where the phase advancingcorrection of the current command is not made in accelerating;

FIG. 4 is a vector diagram in the case where the phase advancingcorrection of the current command is made in accelerating;

FIG. 5 is a vector diagram in the case where the phase advancingcorrection of the current command is not made in decelerating;

FIG. 6 is a vector diagram in the case where the phase advancingcorrection of the current command in decelerating is equal to that indecelerating;

FIG. 7 is a vector diagram in the case where the phase advancingcorrection in decelerating is set to about 1/3 of that in accelerating;

FIG. 8 is a diagram showing the relationship between the motor'srevolutionary speed and the motor output torque in the case where thephase advancing compensation of the current command is made inaccelerating, and is not made in decelerating;

FIG. 9 is a diagram showing the relationship between the motor'srevolutionary speed and the motor output torque in the case where thesame phase advancing compensation is made in both accelerating and indecelerating; and

FIG. 10 is a diagram showing the relationship between the motor'srevolutionary speed and the motor output torque in the case where thephase advancing compensation in decelerating is controlled to about 1/4of that in accelerating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the preferred embodiment, a phase advancing compensation amount δ iscalculated as a linear function of a motor speed v on the basis of thefollowing equation (1).

    δ=K×|v|                      (1)

When the proportional constant of the above linear equation (1) is givenas k in the case of acceleration, the phase advancing compensationamount d is calculated by the following equation (7):

    δ=k×|v|                      (7)

In the case of deceleration of the motor, the proportional constant K isgiven as k' to calculate the amount of phase advancing compensationamount δ by the following equation (8), where k'<k, more specifically,the value of k' is preferably set to about 1/3 of k.

    δ=k'×|v|                     (8)

FIG. 2 is a flowchart of a current command computing section accordingto the preferred embodiment of the present invention in the currentcontrol processing which is executed in every predetermined period(current loop processing period) by the processor of the motorcontroller.

First, in step S1, a value P is determined by multiplying the motorspeed V (including a plus or minus sign indicative of normal or reverserotation) read from an encoder or the like mounted on the motor by thecurrent command Tcmd (including a plus or minus sign indicative ofnormal or reverse rotation command) as P=V×Tcmd, and whether the motoris to be accelerated or decelerated is determined depending on whetherthe value P is positive or negative. When the value P is positive or,zero (0), the motor is considered to be accelerated, and the processingproceeds to step S2, in which the proportional constant K in the linearequation (1) for calculating the phase advancing amount δ is set to k'and stored in register C. However, when the value P is less than zero(0), the motor is considered to be decelerated, and the processingproceeds to step S3, in which the proportional constant K in the linearequation (1) for calculating the phase advancing amount δ is set to k'and stored in the register C. Next, in step S4, the proportionalconstant k or k' stored in the register C is multiplied by the detectedabsolute value of the motor's revolutionary speed v to calculated thephase advancing correction amount δ. Subsequently, in step S5, thecalculated phase advancing compensation amount δ is added to a phase θcorresponding to a rotor position, and a compensated current command(torque command) T is determined by advancing the phase of the currentcommand Tcmd (=To) of each phase. The current loop processing isexecuted on the basis of the corrected current command T to determine avoltage command to be supplied to the motor, whereby the motor is drivenaccording to a PWM control or the like.

FIG. 7 is a vector diagram showing the relationship between command sideand coil side in the case where the phase advancing compensation amountδ of the current command in decelerating is set to about 1/3 of that inthe case of accelerating under the same condition as stated in FIGS. 3through 6. In other words, FIG. 7 shows. a vector diagram in the casewhere the motor is decelerated while being operated at a revolutionaryspeed of 2000 rpm, and the phase advancing compensation amount δ is setto an angle of 37 degrees. As seen from the comparison between FIGS. 7and 6, the power factor is also improved, and the command voltage Vc iswithin the DC link voltage, which is a power-source voltage of theinverter, thereby indicating that an adequate motor output torque can beobtained.

FIGS. 8 through 10 are the diagrams showing the relationship betweenmotor's revolutionary speed and the motor's output torque measured inorder to determine the advantages of the present invention over theprior art.

FIG. 8 shows the measured result of the above relationship in the casewhere the motor is controlled by the phase advancing compensation ofcurrent command only when accelerating the motor, and FIG. 9 shows themeasured result in the case where the motor is controlled by the phaseadvancing compensation, in which the amount of compensation δ iscalculated as d=K×|v| using the same proportional constant K during boththe acceleration and deceleration.

FIG. 10 shows the measured result according to the embodiment of thepresent invention in the case where the aforesaid proportional constantk' in decelerating is set to 1/4 of the proportional constant k used inthe case of acceleration (k'=k/4). FIGS. 8 through 10 show the resultsof experiments conducted under the condition that a rotationary speed ofthe motor is gradually decelerated from a revolutionary speed of -4000rpm to "0" (zero) and then gradually accelerated up to +4000 rpm.

As seen from FIG. 8, in the case where the phase advancing compensationis not applied in decelerating, when the motor is decelerated during ahigh-speed revolution, the motor output torque falls.

As seen from FIG. 9, in the case where the phase advancing compensationis made in decelerating by the same amount as in accelerating, the phasewill be advanced excessively, and the motor's output torque fallstemporarily.

As seen from the measured result shown in FIG. 10, according to theembodiment of the present invention, a sufficient output torque can beobtained from the motor even, during the decelerating.

We claim:
 1. A method of controlling an AC synchronous motor byexecuting current loop processing on the basis of a current command,comprising the steps of:(a) determining a relationship between an actualrevolutionary speed of the motor and a current command; (b) determiningwhether the motor is one of decelerated and accelerated based on therelationship between the actual revolutionary speed of the motor and thecurrent command; (c) advancing a phase of the current command by a phaseadvancing compensation amount according to the determination in step(b), wherein advancing the phase of the current command comprises,(c1)setting the compensation amount to a first value when the motor isdetermined to be accelerating in step (b); and (c2) setting thecompensation amount to a second value smaller than the first value whenthe motor is determined to be decelerating in step (b).
 2. A methodaccording to claim 1, wherein said advancing step (c) furthercomprises:(c3) determining a phase advancing compensation amount by alinear equation proportional to a revolutionary speed of the motor, and(c4) setting a proportional constant of said linear equation indecelerating to a value smaller than that in accelerating.
 3. A methodaccording to claim 2, wherein said proportional constant of said linearequation in decelerating is set to a value ranging within approximately1/4 through approximately 1/3 of a proportional constant inaccelerating.